Mainly there are two tricks.
1.Send FACEBOOK or TWITTER to tollfree number 54321 for atleast ten times continuously.
Repeat the procedure several times a day.Then You will see your airtel sim unblocked.
2.Call 121 or 198 to deactivate Airtel Gprs.Now remove your sim from your phone for two days.After two days put your sim in another gprs enabled phone and dial *567*11#
and activate zero rental plan.
If both the above tricks are not working for you,Remove your sim for 15 days and sim gets unblocked automatically.(Or Avoid using gprs in your phone with your sim for 15 days)